In wireless telecommunication systems information is transferred over the radio interface between a transmitting and/or receiving communication device and a communication network. In General Packet Radio System (GPRS) networks, such as GPRS, Enhanced GRPS (EGPRS) and GSM Enhanced Data rate for Global Evolution (EDGE) Radio Access Network (GERAN) efforts has been made to improve the quality of the data transfer and thus the quality of service (QoS). The term GPRS also relates to EGPRS, GERAN and other enhancements of the GPRS in the present application.
A mobile station (MS), a base station sub-system (BSS) comprising a base transceiver station (BTS) and a base station controller (BSC) including a Packet Control Unit (PCU), and a serving GPRS support node (SGSN) are the main components of the GPRS architecture for communicating between a terminal device and the GPRS network. A gateway GPRS support node (GGSN) enables data transfer between the GPRS network and external data networks, such as an Internet. One or more GGSNs are connected to a SGSN via Internet Protocol (IP) based GPRS backbone network. When the MS communicates with external data networks, the GGSN operates as an IP router between a MS and the external network. Packet data channels (PDCH) are used as physical channels for the packet data transfer in the GPRS. To transfer user data, i.e. other data not produced by the GPRS protocols, a Packet Data Protocol (PDP) context is generated. All messages including PDP context messages are transferred using a Temporary Block Flow (TBF) between a MS and GPRS network.
The Logical Link Control (LLC) protocol is used to transfer data between a MS and a SGSN in the GPRS network. The technical specifications of the LLC layer protocol to be used for Packet Data Transfer (PDT) between a MS and SGSN are defined in the document 3GPP TS 44 064 V 4.3.0 (2002-03) [1]. The LLC layer is independent of the underlying radio interface protocol. The LLC protocol layer consists of Logical Link Management Entities (LLME), Logical Link Entities (LLE) and a multiplex procedure. A LLE is the LLC layer protocol state machine controlling one logical link connection.
The LLC layer operates above a Radio Link Control (RLC) layer on the MS side and above a Base Station Sub-system GPRS Protocol (BSSGP) layer on the SGSN side. Above the LLC layer is a SubNetwork Dependent Convergence (SNDC) protocol layer, that controls the transfer of user data Network layer Packet Data Units (N-PDU) between a MS and SGSN. Also above the LLC layer locates a GPRS Mobility Management (GMM) protocol layer, that uses the services of the LLC layer to transfer messages between a MS and a SGSN.
A MAC (Media Access Control) protocol layer locates under a RLC layer on the MS side. It defines the procedures that enable multiple MSs to share a common radio interface and it allows a MS to use several physical channels in parallel on the MS side of the GPRS network. The MAC also provides the arbitration between multiple MSs simultaneously attempting to access a radio interface.
A RLC/MAC layer protocol of the GPRS is described in the document 3GPP TS 44 060 V4.5.0 (2002-02) [2]. A R-LC/MAC block is a protocol data unit exchanged between RLC/MAC entities, and a RLC/MAC control block is a part of a RLC/MAC block carrying a control message between RLC/MAC entities or RLC data block is a part of a RLC/MAC block carrying user data or signaling data of upper layers. The RLC layer defines the procedures for segmentation and reassembly of LLC PDUs into RLC/MAC blocks and the RLC layer provides also link adaptation. The RLC/MAC is responsible for transmitting LLC PDUs over the radio interface using a Temporary Block Flow (TBF), which is a physical radio connection supporting the unidirectional transfer of LLC PDUs between a MS and the network. A LLC PDU contains user data or GPRS protocol related signaling messages, such as a GMM signaling message (GMM/SM). A MS may have an uplink TBF (UL TBF), a downlink TBF (DL TBF) or an uplink and downlink TBF established at any time. When a transfer mode of LLC PDUs terminates, in either uplink or downlink direction, the corresponding TBF is released and the MS returns to packet idle mode. When a transfer mode of LLC PDUs terminates but there exists an on-going LLC PDU transfer to the other direction, the MS stays in transfer mode.
One TBF may carry RLC data blocks only in one RLC mode at a time. This means that when transferring in-sequence LLC PDUs utilizing different RLC modes (ACK or UNACK), a previous TBF has to be released and a new TBF has to be established for a new RLC mode. The LLC is intended for use with both acknowledged (LLC ACK) and unacknowledged (LLC UNACK) data transfer and the RLC/MAC supports both the RLC ACK mode and RLC UNACK mode. The LLC modes and the RLC modes are independent from each other. In the LLC ACK mode the LLC provides a reliable service with in-order delivery but in the LLC UNACK mode the LLC does not guarantee in-order delivery. In RLC ACK mode RLC uses retransmissions to guarantee error-free transmission and in RLC UNACK mode retransmissions are not used. In both modes RLC/MAC specification says that upper layer PDUs shall be delivered in the order they are received from upper layers. In RLC UNACK mode one lost RLC data block may result in discarding of the whole LLC PDU at the receiving side. In the RLS ACK mode Backward Error Correction (BEC) procedures enable the selective retransmission of unsuccessfully delivered RLC/MAC blocks.
According to the Technical Specifications 3GPP TS 44 064 V 4.3.0 [1] the RLC shall deliver LLC PDUs received from the upper layers in the same order as they were received from the upper layers. This means that LLC PDUs are delivered in the same order as received from the upper layers (i.e. LLC layer), regardless of the fact that some LLC PDUs may have e.g. higher priority than other LLC PDUs. This is a big problem when transferring e.g. real-time or other delay sensitive data over the radio interface, because the data, despite its high priority, have to hold on the transmitting queue of in-order delivery. This may impair the QoS of the application.
The LLC allows data transfer with different service criteria, such that high-priority data transfers may take precedence over lower-priority data transfers to the same MS. A LLC PDU has certain QoS characteristics concerning the RLC mode, priority, throughput, etc. When streaming data or otherwise delay sensitive data, such as speech, is transferred over the GPRS network, it should be delivered before e.g. best effort data, such as FTP (File Transfer Protocol) data or web surfing, to ensure the QoS. Otherwise the service suffers bad quality. Recently an interest towards transferring delay sensitive data over the GPRS network is rising.
An example is now provided to describe the current state of the prior art. Assume that the RLC/MAC of the MS first receives three short LLC PDUs from a delay sensitive application that needs to be transmitted using the RLC UNACK mode. After this the RLC/MAC receives two long, e.g. 1500 octet each, LLC PDUs containing FTP data that needs to be transmitted using the RLC ACK mode. Then after this the RLC/MAC again receives three short LLC PDUs from the delay sensitive application that needs to be transmitted using the RLC UNACK mode. When changing a transfer mode from the RLC UNACK mode to the RLC ACK mode, first an existing TBF is released, then a new TBF is established and then FTP traffic LLC PDUs are transferred in RLC data blocks. After this a transfer mode is changed from the RLC ACK mode to the RLC UNACK mode again by releasing existing TBF and establishing new TBF, and then a transfer of data packets of the delay sensitive application may continue. A time needed to transfer FTP traffic LLC PDUs in the RLC data block depends on the number of assigned uplink PDCHs. The elapsed time also depends on a channel coding scheme used to transfer RLC data blocks over the radio interface and how frequently the TBF is assigned sending permissions. In this example, a transfer of two 1500 octet long LLC PDUs in the RLC ACK mode between the delay sensitive data packets may take several seconds. The gap of several seconds will result in that delay sensitive applications will substantially suffer from the FTP transfer.
The gap of several seconds will result in that a transfer of real time LLC PDUs of applications using streaming or otherwise delay sensitive data will be blocked by a transfer of non-real time LLC PDUs of applications using FTP or other best effort data according to the current specifications. In case of a speech application the quality of the conversation becomes unacceptable. From the foregoing it follows that the current GPRS network is unable to transfer delay sensitive data over the radio interface.
There are significant problems related to prior art to transfer delay sensitive data over the GPRS network. Grounds for the problems in prior art is the fact that a RLC/MAC does not interprete contents of a LLC PDU at all, and it only transfers a LLC PDU, such as received from a LLC, over a radio interface.